Background checks jumped more than 41 percent since Friday's shooting that left 12 dead and 58 injured during a midnight screening of "The Dark Knight Rises" at an Aurora movie theater. Over the weekend, the Colorado Bureau of Investigation approved background checks for 2,887 people who wanted to purchase a firearm, the Post said, an increase of 43 percent over the previous weekend.

"It's been insane," Jake Meyers, an employee at Rocky Mountain Guns and Ammo in Parker, Colo., told the paper.

Spikes in gun sales are not uncommon in the aftermath of mass shootings like the one in Colorado. Following the January 2011 shooting that killed six and wounded more than a dozen others—including former Arizona Rep. Gabrielle Giffords—in Tucson, sales of handguns soared more than 60 percent in the state, according to FBI data. Similar spikes were seen after the massacres at Virginia Tech and Columbine.

Some of those seeking to buy guns in Colorado over the weekend said they were seeking to arm themselves for protection in the wake of the shooting, according to the report. But many were likely fearful of a change in gun laws. Democratic state Rep. Rhonda Fields of Aurora told the paper she wants Congress to reinstate a ban on assault weapons.

"When something like this happens people get worried that the government is going to ban stuff," Greg Wolff, an Arizona gun shop owner, told Bloomberg.com after the rampage in Tucson.

They also get worried when a Democrat is about to take office. Before President Barack Obama's 2008 election, there was a spike in gun sales, and gun shop owners and manufacturers have reported similarly brisk buying in 2012.

"It's definitely the election year," Jason Hanson, a former CIA officer, told Fox News in March. "People feel that Obama will serve second term and with it their gun rights with taken away, so they are stocking up."

In December, the FBI reported a record number of background checks—1,534,414—sent by gun dealers. "Almost half a million checks were done in just the last six days before Christmas," according to CNN. In 2010, the FBI's National Instant Criminal Background Check System received more than 14 million requests, more than in any other year.

James Holmes, the suspected shooter in Friday's massacre, was found with a military-style AR-15 assault rifle, two Glock .40-caliber pistols and a Remington 12-gauge shotgun when he was arrested outside the theater in Aurora. And like Jared Loughner, the accused killer in the Tucson massacre, Holmes purchased the guns legally.